Make headers conform to package.el requirements

I'm adding a [MELPA](http://melpa.milkbox.net/) recipe so that easily-installable packages of `dash-at-point` are available there, but this header fix is required in order for `dash-at-point` to be parseable by the `package-buffer-info` function.

Additionally, I've added an `autoload` cookie comment so that an autoloads file will be automatically created at package installation time, and I've updated the README to reflect the availability of a package in MELPA.

Cheers!

-Steve